Gas discharge lamps such as fluorescent lamps require ballast in order to properly start and maintain lamp ignition to produce adequate light from the lamp. Ballast may be of electromagnetic, electronic or solid state types. With newer lamps, electronic ballast have been required in order to provide the necessary voltage and current to start the lamp and to maintain the required light output.
As a fluorescent lamp ages, several things can occur. For example, an emissive coating on the lamp filament may become depleted to the point the voltage drop from the filament to the arc stream is significantly increased because ionization of the gas in the lamp decreases due to the decrease in filament electron production. This causes the ballast to increase the voltage across the filament in an attempt to increase the current through the lamp in trying to provide the power apparently required by the lamp. As a result, switching devices commonly found in electronic ballast circuits may overheat and fail.
In another example, a lamp may become deactivated, wherein the gas fill of the lamp is either dissipated during use or was not present in sufficient amounts to efficiently fire the lamp. Even though the filaments of the lamp are acceptable, the lamp does not properly fire. The lamp no longer exhibits the necessary resistance to maintain the desirable impedance in the circuit, thereby presenting a relatively low impedance to the ballast. A low impedance permits a relatively high current to be generated in the ballast components, applying a high voltage and current to the lamp filaments. The ballast components operating at such high power levels may overheat and fail.
Some electronic ballast may incorporate circuits to minimize or eliminate the possibility of component damage due to lamp failure. However, such circuits may be relatively expensive, include a relatively large number of components, or may require resetting the ballast before the ballast can again begin operation.